1. Field of the Invention
The present invention relates to magnetoresistive material films for a giant magnetoresistive device employed in magnetic heads, position sensors, rotation as sensors and the like.
2. Description of the Related Art
An NiFe alloy (Permalloy) is a known magnetoresistive (MR) material which has been used for forming thin films for MR devices. Generally, the percentage change in resistance of a Permalloy thin film is within the range of 2 to 3%. Accordingly, magnetoresistive materials having magnetoresistive ratios (MR ratios) greater than that of Permalloy have been desired to cope with increases in linear density and track density in magnetic recording or increases in the resolving power of magnetic sensors.
Recently, a phenomenon called giant magnetoresistive effect has been found in a multilayer thin-film structure, such as a multilayer thin-film structure consisting of alternate layers of Fe thin films and Cr thin films or alternate layers of Co thin films and Cu thin films. In such a multilayer thin-film structure, the magnetizations of the ferromagnetic layers of Fe or Co interact magnetically through the nonmagnetic layers of Cr or Cu and the magnetizations of the stacked ferromagnetic layers are coupled to maintain an antiparallel alignment; that is, in this multilayer thin-film structure, the directions of magnetization of the ferromagnetic layers spaced alternately with the nonmagnetic layers are opposite to each other without an external magnetic field. When an appropriate external magnetic field is applied to such a structure, the magnetization directions of the ferromagnetic layers are aligned in a common direction.
In this multilayer thin-film structure, it is known that the state where the magnetizations of the ferromagnetic layers in an antiparallel alignment and the state where the magnetizations of the ferromagnetic layers are in a parallel alignment differ from each other in the scattering mode of conduction electrons in the interface between the ferromagnetic Fe layers and the nonmagnetic Cr layers or between the ferromagnetic Co layers and the nonmagnetic Cu layers, depending upon the spins of the conduction electrons. Consequently, the electric resistance is high when the magnetization directions of the ferromagnetic layers are in an antiparallel alignment, the electric resistance is low when the magnetization directions of the ferromagnetic layers are in a parallel alignment, which produces the so-called giant magnetoresistive effect causing a resistance change at a high percentage ratio greater than that of resistance change in a Permalloy thin film. Thus, these multilayer thin-film structures have an MR producing mechanism basically different from that of the conventional single NiFe film.
However, since the magnetic interaction between the ferromagnetic layers of those multilayer thin-film structures that acts in an effort to set the magnetizations of the ferromagnetic layers in an antiparallel alignment is excessively strong, a very intense external magnetic field must be applied to those multilayer thin-film structures to set the magnetization directions of the ferromagnetic layers in a parallel alignment. Therefore, a large resistance change cannot be expected unless a very intense magnetic field is applied to the multilayer thin-film structures, and hence magnetic heads that detect an applied magnetic field of a very low intensity created by a magnetic recording medium are unable to function with satisfactorily high sensitivity when such a multilayer thin-film structure is incorporated into those magnetic heads.
It may be effective, for solving such problems, to adjust the thickness of the nonmagnetic layers of Cr or Cu so that the magnetic interaction between the ferromagnetic layers are not excessively strong and to control the relative magnetization directions of the ferromagnetic layers by another means other than the magnetic interaction.
A technique proposed to control the relative magnetization directions of the ferromagnetic layers employs an antiferromagnetic layer, such as an FeMn layer, to fix the magnetization direction of one of the ferromagnetic layers so that the magnetization direction of the same ferromagnetic layer may not be changed by an external magnetic field, and to allow the magnetization direction of the other ferromagnetic layer to change to enable the multilayer thin-film structure to be operated by an applied magnetic field of a very low intensity.
FIG. 22 shows a magnetoresistive sensor disclosed in U.S. Pat. No. 5,159,513 employing the foregoing technique. The magnetoresistive sensor A shown in FIG. 22 is formed by depositing a first magnetic layer 2, a nonmagnetic spacer 3, a second magnetic layer 4 and an antiferromagnetic layer 5 on a nonmagnetic substrate 1. The magnetization direction B of the second magnetic layer 4 is fixed by the magnetic exchange coupling effect of the antiferromagnetic layer 5, and the magnetization direction C of the first magnetic layer 2 is kept perpendicular to the magnetization direction B of the second magnetic layer 4 in the absence of an applied magnetic field. However, since the magnetization direction C of the first magnetic layer 2 is not fixed, the magnetization direction C can be rotated by an applied external magnetic field.
When a magnetic field h is applied to the MR sensor shown in FIG. 22, the magnetization direction C of the first magnetic layer 2 rotates as indicated by the arrows according to the direction of the applied magnetic field h and, consequently, the first magnetic layer 2 and the second magnetic layer 4 become different from each other in magnetization rotation causing a resistance change that enables the detection of the applied magnetic field.
FIG. 23 shows another example of magnetoresistive sensors having the structure in which one magnetic layer has a fixed magnetization direction, the other magnetic layer having a free magnetization direction. As shown in FIG. 23, the MR-sensor B is formed by sequentially depositing an antiferromagnetic layer 7 of NiO, a magnetic layer 8 of Nixe2x80x94Fe, a nonmagnetic metallic layer 9 of Cu, a magnetic layer 10 of Nixe2x80x94Fe, a nonmagnetic metallic layer 11 of Cu, a magnetic layer 12 of Nixe2x80x94Fe, and an antiferromagnetic layer 13 of FeMn in that order on a substrate 6.
In this structure, the antiferromagnetic layers 7 and 13 fix the magnetization directions of the adjacent ferromagnetic layers 8 and 12, and the magnetization direction of the ferromagnetic layer 10 sandwiched between the nonmagnetic layers 9 and 11 and disposed between the ferromagnetic layers 8 and 12 rotates according to the direction of an applied external magnetic field.
In the magnetoresistive sensor having the structure shown in FIG. 22 or 23, the resistance of the magnetoresistive sensor A or B varies linearly with high sensitivity with a small variation of the applied magnetic field. When the first magnetic layer 2 is formed of a soft magnetic material, such as Ni-Fe alloy, the MR sensor has the advantages in that the soft magnetic characteristics thereof can be used, and hysteresis is low.
The magnetoresistive sensor shown in FIG. 22 has the structure in which the antiferromagnetic layer 5 fixes the magnetization of the adjacent second magnetic layer 4, the second magnetic layer 2 having free magnetization. The magnetoresistive sensor shown in FIG. 23 has the structure in which the upper and lower antiferromagnetic layers 7 an 13 of FeMn and NiO, respectively, fix the magnetizations of the ferromagnetic metallic layers 8 and 12 disposed therebetween, the magnetic layer 10 having free magnetization. These magnetoresistive sensors thus have the limitation of the number of the interfaces between NiFe (magnetic layer) and Cu (nonmagnetic metallic layer) and the problem of limiting the MR ratio.
The FeMn alloy used as the material for forming the antiferromagnetic layers 5 and 7 has unfavorable problems with respect to corrosion resistance and environmental resistance.
FIG. 24 shows an example of modification of the magnetoresistive sensors having the structures shown in FIGS. 22 and 23. As shown in FIG. 24, a known magnetoresistive sensor C has a structure in which a nonmagnetic layer 16 of Cu, a hard magnetic material layer 17 comprising Co, Coxe2x80x94Pt or Coxe2x80x94Crxe2x80x94Ta, a nonmagnetic layer 18 of Cu, and a soft magnetic material layer 19 of Nixe2x80x94Fe are deposited repeatedly on a glass substrate 15.
In the magnetoresistive sensor C having the structure shown in FIG. 24, the thickness of the nonmagnetic layer 18 is adjusted to a predetermined value by employing the difference in coercive force between the hard magnetic material layer 17 and the soft magnetic material layer 19 so that the magnetization directions of both magnetic layers 17 and 19 can be made parallel or antiparallel, thereby obtaining the giant magnetoresistive effect. In the magnetoresistive sensor C having the structure shown in FIG. 24, the number of the layers deposited can be freely changed. The MR sensor C thus has the characteristic that the MR ratio greater than those of the magnetoresistive sensors having the structures shown in FIG. 22 and 23 can be obtained by increasing the number of the layers deposited.
However, in the magnetoresistive sensor C having the structure shown in FIG. 24, when the hard magnetic material layer 17 has large coercive force, the MR ratio can be increased, but the leakage flux is increased. This increases the coercive force of the soft magnetic material layer 19, and consequently causes the problem of deteriorating the sensitivity (the rate of change in resistance per unit magnetic field) of the MR sensor.
As a result of repeated research for solving the above problem, the inventors reached the conclusion that the high coercive force of the hard magnetic material layer 17 affects the soft magnetic material layer 19 because the anisotropic dispersion of the soft magnetic material layer 19 is increased due to the leakage magnetic field from the soft magnetic material layer 19. The inventors also reached the conclusion that the high coercive force affects the soft magnetic materia layer 19 because the reversal of magnetization of the soft material material layer 19 is inhibited by the hard magnetic material layer 17 due to the magnetostatic coupling between the hard magnetic material layer 17 and the soft magnetic material layer 19.
Therefore, such a magnetoresistive sensor C has the problem of making it difficult to adjust the effect of the coercive force of the hard magnetic material layer 17 on the soft magnetic material layer 19, and control the remanence ratio.
The present invention has been achieved in consideration of the above-mentioned situation, and an object of the present invention is to provide a giant magnetoresistive material film, a method of producing the same and a magnetic head using the same which can be formed without using a material such as FeMn having the problems with respect to corrosion resistance and environmental resistance, so as to have excellent corrosion resistance and heat resistance, cause no current shunt loss and the giant magnetoresistive effect by employing a difference in coercive force between magnetic layers, which permits the use of similar materials for forming magnetic films having a difference in coercive force and a decrease in coercive force of a film showing the rotation of magnetization, and which can obtain a large change in resistance with a small magnetic field.
In order to achieve the object, in accordance with an aspect of the present invention, there is provided a giant magnetoresistive material film comprising at least two ferromagnetic layers which are formed on a substrate through a nonmagnetic layer, wherein the direction of magnetization of at least one of the ferromagnetic layers is fixed by a coercive force increasing layer provided adjacent to the ferromagnetic layer to increase coercive force, the other ferromagnetic layer having free magnetization so that a change in resistance occurs at a low magnetic field.
In the above structure, the coercive force increasing layer may comprise an antiferromagnetic material, an antiferromagnetic oxide or xcex1-Fe2O3.
It is preferable that the coercive force of the ferromagnetic layer adjacent to the coercive force-increasing layer is set to 50 to 2000 Oe, the coercive force of the ferromagnetic layer not adjacent to the coercive force-increasing layer being set to 0 to 40 Oe. It is more preferable that the coercive force of the ferromagnetic layer adjacent to the coercive force-increasing layer is set to 100 to 1000 Oe, the coercive force of the ferromagnetic layer not adjacent to the coercive force-increasing layer being set to 0 to 20 Oe.
In the above-described structure, the coercive force-increasing layer comprising xcex1-Fe2O3 is preferably formed by sputtering under Ar gas pressure of 3 mtorr or less.
The thickness of the coercive force-increasing layer comprising xcex1-Fe2O3 is preferably set to 200 to 1000 xc3x85.
Each of the ferromagnetic layers preferably comprises an Nixe2x80x94Fe alloy, Co or an Nixe2x80x94Fexe2x80x94Co alloy. It is also preferable that the ferromagnetic layer adjacent to the coercive force-increasing layer comprises Co, and the ferromagnetic layer not adjacent to the coercive force-increasing layer comprises an Nixe2x80x94Fe alloy layer or a two-layer structure comprising a Co layer adjacent to the nonmagnetic layer, and an Nixe2x80x94Fe alloy layer.
The ferromagnetic layer adjacent coercive force-increasing layer preferably has a thickness of 150 xc3x85 or less.
On the other hand, the nonmagnetic layer preferably comprises at least one selected from Au, Ag, Cu and Cr. The nonmagnetic layer preferably has a thickness of 10 to 50 xc3x85, and more preferably a thickness of 20 to 30 xc3x85.
In accordance with another aspect of the present invention, there is provided a method of producing a giant magnetoresistive material film comprising the steps of forming a coercive force-increasing layer comprising xcex1-Fe2O3 on a substrate by sputtering with applying a magnetic field in an atmosphere under Ar gas pressure of 3 mTorr or less, and forming at least two ferromagnetic layers on the coercive force-increasing layer with a nonmagnetic layer therebetween.
In accordance with still another aspect of the present invention, there is provided a method of producing a giant magnetoresistive material film comprising the steps of forming a coercive force-increasing layer comprising xcex1-Fe2O3 on a substrate by sputtering without applying a magnetic field in an atmosphere under Ar gas pressure of 3 mTorr or less, and forming at least two ferromagnetic layers on the coercive force-increasing layer with a nonmagnetic layer therebetween.
In accordance with a further aspect of the present invention, there is provided a magnetic head comprising lower and upper core layer opposite to each other with a magnetic gap therebetween, a coil layer for applying a magnetic field to the core layers, a giant magnetoresistive device provided adjacent to the magnetic gap outside of the core layers, wherein the giant magnetoresistive device comprises the above-described magnetoresistive material film.